This invention relates to the control of a freight lift of the kind attached to a back part of a freight truck for transporting freight up into and down from the truck. More particularly, the invention relates to a mechanism and a method for controlling the upward and downward translational motion of its freight-receiving table (herein referred to as "the tail gate").
FIGS. 2A and 2B show a lift of the type related to this invention, attached to a back part of a freight truck. Such a lift generally makes use of a mechanism comprising a plurality of lift cylinders for moving the tail gate up and down translationally as a whole and a plurality of tilt cylinders for rotating it around a horizontal axis between its opened and closed positions.
FIG. 3 shows a portion of a gate-lifting mechanism 30 of such a lift, comprising a tail gate 31, a lift cylinder 32 with a piston, a main arm 36 and an attachment bracket 38. One end of the lift cylinder 32 is axially connected to the main arm 36 around a pin 34, the other end being axially attached to the attachment bracket 38. Similarly, one end of the main arm is axially connected to the tail gate 31 around a pin 33, the other end being axially attached to the attachment bracket 38 around a pin 35. The attachment bracket 38 is welded to the chassis 37 of the truck.
The lifting motion (both upward and downward) of the tail gate 31 as a whole is effected by a parallelogram link mechanism formed by pins 33, 33', 35 and 35'. Although not shown in FIG. 3 for the sake of clarity, a tilt cylinder for causing the tail gate 31 to rotate (or tilt) around the pin 33 is axially supported between the pins 33' and 35'. While the tail gate 31 is being lifted up and down, the tilt cylinder keeps its length constant and serves, together with the main arm 36, as a part of a parallel link. When the tail gate 31 is lifted up as a whole (as opposed to being lifted up by rotating around the pin 33), a hydraulic liquid (referred to as "the oil") is introduced from a hydraulic pump into one of the oil chambers inside the lift cylinder 32, causing its piston to be extended outward, such that the tail gate 31 is lifted gradually from the position (b) to the position (a) indicated in FIG. 3 while maintaining the parallel link. When the tail gate 31 is lowered as a whole, the oil inside the oil chamber of the lift cylinder 32 is returned to the hydraulic pump, causing the pressure inside the cylinder to drop. The piston of the lift cylinder 32 is thereby returned to its original position, and the tail gate 31 moves down from the position (a) to the position (b), again by maintaining the parallel link. In summary, the up-and-down lifting motion of the gate-lifting mechanism 30 is effected by the reciprocating motion of the piston of the lift cylinder 32.
This operation of the lift cylinder 32, as described above, is controlled by a hydraulic pump, a power unit which may include an electric motor for driving this hydraulic pump as well as solenoid valves, and an electrical control system.
With a prior art lift of this type, an accurate control of its up-and-down motion was difficult because the speed of motion of the tail gate was significantly affected by the load thereon. As a result, the motion of the tail gate was abrupt, starting and stopping with a jerk, causing damage to the cargo being transported thereon. When the tail gate is lowered, furthermore, it tended to hit the ground with a large force which would damage the tail gate itself, affecting adversely the useful lifetime of the equipment.
In addition, a prior art lift of this type had to be operated manually when the tail gate was to be stopped in the middle of its trajectory. This means that the motion of the tail gate had to be started and stopped many times until it reaches a desired height. It also goes without saying that abrupt starting and stopping of the tail gate are not desirable practice from the point of view of safety to the workers.